Lipids and proteins associate covalently to form lipid-linked proteins and noncovalently to form lipoproteins. The lipid portions of lipid-linked proteins anchor their attached proteins to membranes and mediate protein-protein interactions. Proteins form covalent attachments to lipids in several ways, one of which is the covalent attachment of isoprenoid groups, mainly the C.sub.15 farnesyl and C.sub.20 geranylgeranyl residues.
In mammals, geranylgeranyltransferases is known to catalyze the transfer of a geranyl-geranyl moiety from geranylgeranyl pyrophsophate to both cysteines in Rab proteins (Farnsworth, C. C. et al. (1994)Proc Natl Acad Sci USA 91 (25):11963-11967) Rab proteins are Ras-related small GTPases that are geranylgeranylated on cysteine residues located at or near their C termini. Mammalian protein geranylgeranyl transferases types 1 and 2 are heterodimers composed of an alpha and beta subunit. The alpha subunit shows homology to the alpha subunits of a closely related enzyme, farnesyltransferase.
Farnesyltransferases have been described in pea, tomato, and Arabidopsis, but have not been described in monocots. The plant farnesyltransferases also consist of alpha and beta subunits. The geranylgeranyl transferase beta subunit belongs to the protein prenyltransferase beta subunit family. The beta subunits of the type 1 and 2 geranylgeranyltransferases have not been previously described in plants. Work done in yeast has established that geranylgeranyltransferases are distinct from the closely related farnesyltransferases.
The mammalian geranylgeranyl transferases require the aid of a RAB escort protein (also called component A). RAB escort protein is required for Rab geranylgeranyl transferase activity in mammals. RAB escort protein binds unprenylated RAB proteins, presents it to the catalytic component B (alpha/beta subunit complex of geranylgeranyltransferase). RAB binding protein remains bound to the prenylated protein after the geranylgeranyl transfer reaction. Component A may be regenerated by transferring its prenylated RAB to a protein acceptor.
There is a great deal of interest in identifying the genes that encode geranylgeranyl transferase in plants. These genes may be used in plant cells to control cell growth. Accordingly, the availability of nucleic acid sequences encoding all or a portion of geranylgeranyl transferase proteins would facilitate studies to better understand cell growth in plants, provide genetic tools to enhance cell growth in tissue culture, increase the efficiency of gene transfer and help provide more stable transformations. Geranylgeranyl transferase proteins may also provide targets to facilitate design and/or identification of inhibitors of cell growth that may be useful as herbicides.